1. Field of the Invention
The present invention relates to an image pickup device that realizes receiving-light sensitivity characteristic that is close to the sensibility of human vision, and more particularly, to an image pickup device that can detect sufficient contrast in an entire region even if there is an area with a large difference in luminance present in a field of view. Hence, the inventive technique can be utilized in various situations for an imaging device having high visual sensibility, such as a monitoring camera for an area including outdoors, a camera that is mounted on a car and the like.
2. Description of the Background Art
Solid-state imaging devices such as CCD (Charge-Coupled Device) and CMOS (Complementary Metal-Oxide Semiconductor) imagers, i.e. semiconductor image sensors (hereinafter also referred to as “image pickup device”), have been embedded in video cameras, digital cameras, and nowadays even in a mobile phone, and have become widespread as inexpensive imaging devices with low power consumption.
The sensing capability of an image pickup device, however, is much inferior to the sensibility of human vision. Human vision can sufficiently detect contrast in brighter and darker portions even if there is a luminance distribution ranging over approximately four to five digits. This excellent capability in contrast sensing is realized by such a function that each of photoreceptive cells in retina can individually adjust its receiving-light sensitivity characteristic per cell.
By contrast, in a conventional image pickup device, all pixels had the same receiving-light sensitivity characteristic, which made it difficult to obtain sufficient contrast in both brighter and darker portions in the field of view at the same time.
FIGS. 14 and 15 are the first and second views showing examples of images shot by the conventional image pickup device.
Referring to FIGS. 14 and 15, each of these image examples shows a scene shot in a room lighted with a fluorescent in the daytime on a sunny day, containing the outside of a window. Such a scene is commonly seen in everyday life. It is clear from experience that a human eye would have no difficulty in seeing both the inside and outside of the room with sufficient contrast.
In the conventional image pickup device, however, when a receiving-light sensitivity characteristic is adjusted to allow the inside to be seen, the outside of the window becomes too bright, so that contrast in a brighter portion cannot be detected, as shown in the example shot in FIG. 14. On the contrary, when the sensitivity is so adjusted as to allow the outside to be seen with sufficient contrast, as shown in FIG. 15, the inside of the room now becomes too dark, preventing detection of contrast in a darker portion. This reveals that the conventional image sensor is much inferior to human vision in detection of sufficient contrast in the entire region when there are brighter and darker portions present at the same time in the field of view.
The low contrast sensibility is one of major problems to be solved in order to practically use the conventional image pickup device as a device for detecting visual information that substitutes for human vision.
The low contrast detecting capability of the conventional image pickup device will now be described with reference to FIG. 16.
In general, illuminance of the outdoors in the daylight is approximately several tens of thousands of lux. By contrast, illuminance in a room lighted with a fluorescent is approximately 500 lux at a portion not exposed to sunlight, and is 100 lux or lower at a darker portion. That is, as can be seen in the example of luminance distribution in FIG. 16, the example shots shown in FIGS. 14 and 15 exhibit the difference of two to three digits between the average luminance of the outside of the window and that of a dark portion in the room (the face portion).
In the example shot shown in FIG. 14, the receiving-light sensitivity characteristic of pixels is adjusted to have a sensitivity line shown in FIG. 16, such that sufficient contrast can be detected in a relatively bright region (at most 500 lux) in the room. In the conventional image pickup device, however, all pixels have the same sensitivity characteristic as described earlier. Hence, with the same sensitivity characteristic, a bright portion outside the window will be white due to saturation of the sensitivity. Moreover, in a dark portion, contrast will also be insufficient, since the amplitude is lowered to one-fifth of the whole. Thus, the conventional image pickup device cannot detect sufficient contrast even in a scene we often come across in everyday life.